1. Field of the Invention
The present invention relates to direct and computed radiography.
The invention more particularly relates to a method for preventing a sub-optimal gain map quality by detecting avoidable disturbances present in the x-ray beam-path during system (re)calibration.
2. Description of the Related Art
System calibration is extremely important for digital and computed projection radiography where flat-panel detectors and x-ray storage media in combination with digitizers are used to acquire digital images for clinical, veterinary or industrial use.
These image acquisition devices are rather complex hybrid (analog and digital) systems which are composed of a variety of highly interacting mechanical, electro-optical, physico-chemical, electronics, software and image-processing components and processes each having its typical tolerances and physical properties.
The overall image quality performance of a radiographic system can also depend on the ambient temperature, the humidity, the atmospheric pressure as well as on the x-ray exposure history linked to the degree of system usage and the system's actual age.
In addition gradually increasing levels of system contamination due to the external contact of the system components with radiographed patients, animals, objects, operator personnel or caused by fiber- and dust particle pollution can depend on the equipment's application-specific usage modes and the ambient climate conditions and can influence the properties and the behaviour of the system's individual components and processes thus leading to more frequent, beyond the periodically scheduled, cleaning and recalibration activities.
Especially for the flat-panel-detectors where next to dense pixel-individual light-trapping or direct x-ray detection array-circuitry also massive amounts of highly miniaturized pixel-, row-, and column-specific galvanic interconnections and several block-wise arranged read-out electronic circuits should cooperate harmonically the signal-quality and the x-ray response of the individual image-pixels, the image-rows and the image-columns will slowly or sometimes even suddenly degrade to a level where the required high image quality level can no longer be assured without corrective actions.
Therefore the image-acquisition system needs to be cleaned and recalibrated on a regular basis.
These activities are typically executed not only after the equipment is delivered and installed as a precondition to performing its initial acceptance test but also before each scheduled periodic quality control test.
After an equipment move, after system modifications and after preventive maintenance or repair interventions to critical system-components an additional system recalibration is often necessary as well to ensure a safe and effective operation of the radiographic image-acquisition system within the predetermined overall image quality range.
The system calibration process not only delivers a better adjusted and cleaner state of the radiographic equipment but also generates one or multiple image-wide maps at pixel resolution for the reconstruction of unstable and or defective pixels, rows and columns in addition to one or more gain maps for the software- or hardware-based, pixel-wise sensitivity-correction of raw diagnostic images. A gain map of a detector system is an image-wide representation of the (relative) signal response of each individual detector-pixel to x-ray dose.
Once established these freshly generated correction maps are used to optimize the image quality of each raw image acquired from then on and these maps will remain unchanged and in effect till the next system recalibration process, which will produce a new set of correction maps, is performed successfully.
In general several thousands of raw flat-panel-detector images acquired are corrected using the same set of correction maps, determined during the last system (re-)calibration.
(Re-)Calibration activities are, although of vital importance for a normal system operation, rather workflow-disturbing since the process of system-cleaning, equipment re-adjustments, calibration-dedicated image set acquisition and data-processing to generate the correction maps often requires the manual intervention of an operator and can be time-consuming while making a normal diagnostic use of the system impossible.
Without regulations or local recommendations on the minimum frequency of system recalibration the argument of the inevitable down-time due to system recalibration activities can lead to situations where users tend to postpone the system recalibration process as long as possible.
This by consequence means that even more than the normally already high amount of raw flat-panel-detector images acquired will be corrected based on that same set of correction-maps.
If the gain map, used for the pixel-wise sensitivity correction of the raw images acquired, would slowly degrade over time to a state where it becomes insufficiently representative, a vast amount of corrected diagnostic images might be impacted by various levels of image-artifacts of which some could be only faintly noticeable after a while and this might lead to deteriorated reading comfort, radiologist uncertainty and eventually to a false diagnosis.
Even an image-retake, causing a patient or object to be re-exposed in addition to introducing a time-costly workflow-disturbance, might in such cases still be insufficient to make up for the locally lacking, disturbed image quality of the corrected image since the same suboptimal gain map, the real cause of the problem, will again be used to correct the new image.
The gain map, which is determined as one of the outputs of the (re)calibration process, is often calculated from a set of non-x-ray exposed, raw dark-images in combination with a set of dedicated, homogeneously exposed raw flat field images.
Once the tube's focal spot, the source-to-image distance, the level of beam-collimation, the beam-filtration, the tube-voltage, the tube current and the exposure-time are set up and also the flat-panel-detector is geometrically positioned these dedicated image sets, required for the gain map determination during calibration, can easily be acquired sequentially from the operator's control cabinet without the need for further manual interventions to the system itself.
Sometimes though, visible objects that are forgotten to be removed (e.g. a dosimeter, a cleaning cloth, tools, all other kinds of objects commonly used by operators and service personnel during maintenance, repair and recalibration) as well as invisible objects (falsely positioned collimator blades, a badly mounted grid or automatic exposure control chamber) can still disturb the x-ray beam path during the actual acquisition of the calibration-dedicated images. These beam-path disturbance problems will not likely be seen from that remote operator location given that these individual images are acquired semi-automatically without any visual inspection performed on them at all.
These disturbing objects can be size-wise difficult to detect and can partially absorb the x-ray radiation towards the flat-panel detector-surface thus locally impacting the homogeneous character of the exposure field, required for the successful acquisition of the set of representative flat field images for the calculation of the gain map.
Disturbing objects can have various dimensions from very big (e.g. a screw-driver, a pull-over) to very small (e.g. a screw, a washer, a lost staple).
Some objects with a mixed material composition can locally introduce strongly fluctuating x-ray attenuation (e.g. a dosimeter) whereas others have hardly noticeable, fuzzy and noisy object-borders (e.g. a cleaning-cloth). Even an unexpected visit of an insect accidentally interfering with the x-ray beam path during system recalibration can't be totally excluded.
In addition non-corrected raw flat-panel detector images, also in scope for this inspection, can exhibit a significant level of streakiness and strip-wise signal-variation due to the multiple ASICs-based electronic circuits used for the parallel image read-out of the detector array.
An inspection concept relying on the signal difference between an object and its smooth background or on signal-gradient-based edge-analysis will be insufficiently effective to look for all these kinds of image-disturbances given the presence of the other non-object related noise sources inherent to raw, non-corrected images.
If no disturbing object inspection on each of the raw flat field exposed images is performed to upfront determine whether or not that image is sufficiently disturbance free and can be allowed to act as a valid and representative input image for the calculation of the gain map, sub-optimal gain maps for the correction of many thousands of future raw diagnostic images can potentially be generated without notice.
Best case such a sub-optimal gain map will generate a corrected verification flat field image, often acquired as a final step in the calibration procedure, which is showing a sufficient level of image disturbance to be regarded as a system calibration problem by the operator who is performing the (re)calibration activities.
In that case the calibration must be regarded as failed and a new calibration process with the acquisition of new flat field images is required.
Re-performing the system-calibration requires additional work and can significantly increase the system's down-time.
The problem originating from a disturbed gain map passes the calibration process undetected in case no final flat field verification, using the new but object-disturbed gain map for correction of the raw flat field image, is performed.
It will then depend on the radiologists experience and on the level of disturbance visibility whether or not that sub-optimal calibration state, generating disturbed diagnostic images after raw image correction, can be either detected and mitigated quickly by a system-recalibration or will inevitably lead to a reduced overall image quality level which might hamper the proper diagnoses of many corrected images to follow.
Although the fact that the acquisition of gain correction data should be done in the absence of any object in the X-ray beam irradiating the detector is well-known from prior art references EP 2 050 395: WO 2007/043974 and DE 10 2005 017491, none of the prior art references addresses the non-trivial problem with disturbing objects which may lead to imperceptible acquisition of corrupted gain correction data.